46 A YEAR IN SCIENCE 



is a gas, may be condensed into a liquid form by great 

 pressure, and as soon as the pressure is removed the 



Fig-. 24. Interior of tank room in ice factory. The brine tank 

 and the ammonia coils are under the floor. Two cans ready to be 

 filled with distilled water are shown at the right. 



liquid quickly changes into a gas again, the rapid 

 change requiring much heat. 



Ammonia gas is liquefied by strong pressure and low 

 temperature. It is then allowed to flow into pipes 

 which run through tanks containing salt water. As 

 soon as the pressure is reduced, the liquid ammonia 

 evaporates and becomes a gas. In doing this it with- 

 draws heat from the salt water, reducing its tempera- 

 ture far below the freezing temperature of pure water. 

 Immersed in the salt water are molds containing pure 

 water. The water in the molds freezes and is with- 

 drawn as solid cakes of ice. 



Heat given off in condensation. If a glass plate is 

 held over boiling water, drops of water collect on it. 

 The steam is cooled on coming in contact with the glass 



